Heterosigma akashiwo is a raphidophyte known for forming ichthyotoxic blooms. In order to predict the potential impacts of rising CO2 on H. akashiwo it is necessary to understand the factors influencing growth rates over a range of CO2 concentrations. Here we examined the physiology and gene expression response of H. akashiwo to concentrations from 200 to 1000ppm CO2. Growth rate data were combined from this and previous studies and fit with a CO2 limitation-inhibition model that revealed an apparent growth optimum around 600-800ppm CO2. Physiological changes included a significant increase in C:N ratio at ?800ppm CO2 and a significant decrease in hydrogen peroxide concentration at ?1000ppm. Whole transcriptome sequencing of H. akashiwo revealed sharp distinctions in metabolic pathway gene expression between ?600 and ?800ppm CO2. Hierarchical clustering by co-expression identified groups of genes with significant correlations to CO2 and growth rate. Genes with significant differential expression with CO2 included carbon concentrating mechanism genes such as beta-carbonic anhydrases and a bicarbonate transporter, which may underpin shifts in physiology. Genes involved in cell motility were significantly changed by both elevated CO2 and growth rate, suggesting that future ocean conditions could modify swimming behavior in this species.